1. Field of the Invention
The present invention relates to an SAW (Surface Acoustic Wave) filter and in particular, to a resonator type SAW filter.
2. Description of the Prior Art
A SAW filter has been generally used as a high band pass filter. Since the SAW filter is structured in a smaller size than an LC filter, the SAW filter has been used in many portable electronic apparatuses such as portable telephone units.
As a first prior art reference, JPA-63-92123 discloses some SAW filters as shown in the accompanying FIG. 6 thereof which are composed of four resonance elements of which each comprises a pair of interdigital electrodes and reflectors that are disposed beside both sides of the pair of interdigital electrodes and reflect a surface wave excited by one of the interdigital electrodes. The resonance element is shown in FIG. 1 thereof. Because the combinations of resonance elements are suitably selected, the SAWs of the first reference has filter characteristics which cannot be obtained by each of the resonance elements.
As a second prior art reference, JPA-5-251987 discloses some internally impedance-matched acoustic surface wave filters which are shown in the accompanying FIGS. 1 and 3. The filters of the second reference also comprise four resonance elements and have advantages of small insertion loss and small attenuation.
As a third prior art reference, JPA-7-254835 discloses a longitudinal mode type filter which comprises two reflectors and a pair of interdigital electrodes disposed between the reflectors as shown in the accompanying FIG. 1. A surface wave excited by the input electrode is enclosed between the reflectors and effectively received by the output electrode. Thus, the loss of the filter is small.
In FIG. 8 of JPA-63-92123 as the first reference, a combinatorial filter making use of a longitudinal mode resonance is disclosed.
Although the filters of the prior art references have the above-explained advantages, the filters have disadvantages of strong spurious response and small out-band attenuation. On the other hand, a transversal mode coupled type resonance filter has advantages of weak spurious response and large out-band attenuation. Therefore, the transversal mode coupled type resonance filter has been widely used. As shown in FIG. 7, the transversal mode coupled type resonance filter has a first resonator which is the upper half portion of FIG. 7 and a second resonator which is the lower half portion of FIG. 7. The first resonator has interdigital electrode 16 and reflectors 18 and 19 disposed beside both sides of interdigital electrode 16. Reflectors 18 and 19 reflect a surface wave excited by interdigital electrode 16. The second resonator has the same structure as the first resonator including interdigital electrode 17. The first and second resonators are adjacently disposed in parallel so that their propagating direction of a surface wave become identical. An input ground electrode and an output ground electrode are structured in common as common bus bar 20. Since the two resonators are adjacently disposed, surface waves enclosed between the reflectors of the resonators have a symmetrical resonance mode and an asymmetrical mode. In the symmetrical resonance mode, the energy distribution perpendicular to the propagating direction of the surface wave is symmetrical. In the asymmetrical resonance mode, the energy distribution perpendicular to the propagating direction of the surface wave is asymmetrical. The two modes are referred to as transversal modes. When an interdigitated width of the interdigital electrodes and a thickness of thin film of the interdigital electrodes are properly selected, symmetrical mode S0 or asymmetrical mode A1 can be selectively excited as shown in FIG. 3. FIG. 8 shows an example of a frequency characteristic of such a filter. In FIG. 8, a dashed line represents a curve measured with a 50-ohm system, whereas a solid line represents a curve in a impedance-matched state. As shown in FIG. 8, the mode S0 is present at a lower frequency than the mode A1. When this filter is used in an impedance-matched state, a filter characteristic with a flat band as represented by the solid line can be obtained.
As explained above, a bandwidth of this filter depends on the difference between the two resonance frequencies. The difference between the two resonance frequencies depends on distance G between the two resonators and interdigitated width W of each resonator. In order to widen the bandwidth, distance G and the interdigitated width W should be decreased. However, when the interdigitated width W of the resonators is decreased, the impedance of the filter is increased. Thus, the usefulness of the filter deteriorates. In order to decrease distance G between the resonators, it is necessary to decrease a width of common bus bar 20. Thus, a resistance of bus bar 20 increases, whereby the insertion loss characteristic of the filter deteriorates, and moreover, the frequency characteristic of the entire filter occasionally and remarkably deteriorates. Consequently, it was difficult to widen the bandwidth of the conventional transversal mode coupled type filter.